The present invention relates to a magnetic head controlling apparatus employed in the magnetic disk apparatus to control the seeking operation of the magnetic head.
With the magnetic disk apparatus, it is after the position determining operation of the magnetic head (which will hereafter be referred to as the head) is carried out relative to a magnetic disk platter (which will hereafter be referred to as the disk) that the reading and writing operations are conducted relative to data at a certain position on the disk. The position determining operation is usually called the seeking operation. Generally speaking, the manner of controlling the seeking operation includes speed control at the time when the head is moved to an intended track on the disk and position control at the time when the head is positioned accurately on the intended track. In addition to these speed and position controls, there is also included transient control at the time when the speed control is shifted to the position control.
Upon controlling speed in the course of the seeking operation control, the distance which the head is to move to an intended track on the disk is calculated by a moving distance calculator circuit 10 shown in FIG. 1. An address TA, which represents the intended track, is instructed by a controller of the magnetic disk apparatus and the calculator circuit 10 calculates the moving distance of the head on the basis of this address TA. The present position of the head on the disk is stored in the apparatus. Based on a position signal A provided by a position detector circuit (not shown), the track detector circuit 11 generates a track pulse TP when the head passes a track on the disk. The position detector circuit provides the position signal, which represents the position of the head, on the basis of servo-data previous-ly, recorded on the disk. A velocity detector circuit 12 detects the moving velocity Vd of the head on the basis of the position signal A.
Both of the track pulses TP applied from the track detector circuit 11 and the moving distance data (or number TN of tracks crossed) of the head applied from the moving distance calculator circuit 10 are supplied to a velocity data calculator circuit 13, which calculates an intended velocity data (or number of tracks) V1 of the head on the basis of the track pulses TP and number of crossed tracks TN generated when the head actually moves. A velocity signal generator circuit 14 is provided with a table where the velocity signal, which corresponds to the intended velocity data V1 applied from the velocity data calculator circuit 13, is previously stored as a digital data. The velocity signal generator circuit 14 converts the digital data selected from the table to an analog signal, using a D/A converter, and generates an intended velocity signal V2.
A subtracter circuit (which usually consists of operational amplifiers) supplies to a carriage an output signal D, which is equal to the difference between the intended velocity signal V2 applied from the velocity signal generator circuit 14 and the detected velocity signal Vd applied from the velocity detector circuit 12 and corresponding to the moving velocity of the head, whereby the carriage is driven to make the detected velocity signal Vd equal to the intended velocity signal V2. Namely, the head is moved to the intended track on the disk at a predetermined speed thanks to the drive of the carriage.
The velocity data calculator circuit 13 operates in such a way that the number of crossed tracks (or data corresponding to the intended moving distance) TN is reduced in response to the track pulse TP, generated every time the head crosses a track on the disk. The intended velocity signal V2 is produced in the velocity signal generator circuit 14 on the basis of the intended velocity data (or number of tracks) V1 applied from the velocity data calculator circuit 13. This intended velocity signal V2 is obtained from an equation where velocity is usually proportional to the square root of distance. It is assumed here that the intended velocity signal V2 changes, as shown by a curve 20 in FIG. 2A, for example, as the head moves on the disk. Now, providing the head starts moving from a point D and stops at a point A in FIG. 2A, the difference in this case between the intended velocity (or curve 20) and the actual moving velocity (or curve 21) of the head is substantially high, i.e., starting from point D and coming to point E. Therefore, output of the subtracter circuit 15 shown in FIG. 1 is large enough to supply a large drive current to the carriage. Namely, acceleration current I+, represented by numeral 23 in FIG. 2B, for example, is supplied to the carriage. The velocity of the head is thus increased like the curve 21 in FIG. 2A, and when the head reaches its intended velocity at point E, it moves at a certain velocity without being accelerated and decelerated.
When the head comes near the stop position (or intended track) A, the intended velocity is reduced to achieve the decelerating operation. The deceleration current (or acceleration current I+ and reversed polarity) I-, represented by numeral 24 in FIG. 2B, for example, is supplied to the carriage this time. In the case of a short seeking operation where the head moves from point B to point A in FIG. 2A, a certain velocity section vanishes and drive currents, represented by numerals 25 and 26 in FIG. 2C, for example, are supplied to the carriage.
The drive current supplied to the carriage is changed corresponding to the moving distance of the head, as described above, to control the velocity of the head. However, the drive current supplied to the carriage becomes that which is represented by broken lines in FIGS. 2B and 2C due to the properties of the carriage and its drive circuit. This causes the changeover of the head from acceleration to deceleration to be delayed, thereby erroneously making the moving velocity of the head different its than intended velocity. If the seeking operation of the head is carried out over a relatively long distance in this case, the error will become smaller as time lapses. Until the head reaches point A (or the intended track), therefore, the moving velocity of the head can substantially follow the intended velocity thereof.
When the seeking operation of the head is short (or when the head moves from point B to point A in FIG. 2A, for example), however, the delay of deceleration influences the moving velocity of the head until the head comes just before point A. In the case of a short seeking operation, therefore, it happens sometimes that the velocity of the head unnecessarily increases when velocity control is changed over to position control (or transient control) just before the intended track (or point A). A seeking mistake is thus caused in the worst case, thereby lowering the accuracy for determining the position of the head at the time when the head is being positioned on the intended track.
In order to solve this problem, it is imagined that the intended velocity (or curve 20 in FIG. 2A) is previously set low, but the moving time of the head during the seeking operation is increased in this case. Namely, the average value of seeking time increases to conventionally carry out the short seeking operation (in which the moving distance of the head is small) with high accuracy. As a result, it is difficult to realize a high velocity operation of the magnetic disk apparatus.